The present invention relates generally to arc furnaces used in the making of steel. More particularly, this invention pertains to AC arc furnaces having an auxiliary system to control undesirable arc deflection within the furnace.
In a typical AC arc furnace used in steel making operations, three graphite electrodes are vertically positioned as columns within a cylindrical furnace shell. The upper ends of the electrodes are mechanically secured to electrode terminals and are each electrically connected in a xe2x80x9cdeltaxe2x80x9d arrangement to a three-phase AC electrode power circuit exterior to the shell. In response to a conventional regulation and control circuit associated with the electrode power circuit, arcs are generated between the electrode tips and conductive material placed at the bottom of the furnace. This begins the heating, melting, and refining processes familiar to those who design and operate AC arc furnaces for use in the making of steel.
One problem that is well known to engineers who design AC arc furnaces is arc deflection caused by magnetic fields present inside the furnace. These internal magnetic fields are generated by electromagnetic interaction of the current flowing through each of the electrode current loops formed by the electrode power circuit, electrode, arc, and conductive material at the bottom of the furnace. Arc deflection, in which the arcs are caused to extend outwardly from the electrodes at an angle rather than vertically in line with the electrode columns, has several undesirable effects on furnace operation. For example, arc deflection, sometimes referred to as arc blowout or arc flare, moves the arc laterally toward the furnace shell. This results in an uneven heat distribution inside the furnace, producing what are commonly known as xe2x80x9chot spotsxe2x80x9d and xe2x80x9ccold spotsxe2x80x9d. In some cases, auxiliary burners must be used to compensate for cold spots and energy is lost in furnace water-cooling systems from the hot spots. Arc deflection also produces non-uniform melting of the material inside the furnace which can cause scrap xe2x80x9cfallsxe2x80x9d and electrode breakage.
There have been attempts made in the prior art to use externally generated magnetic fields as a means of controlling arcs inside an arc furnace. For example, in U.S. Pat. No. 4,034,146 issued to Stenkvist in 1977, an external electromagnetic coil is positioned underneath a three-phase AC arc furnace. The coil generates a (DC) magnetic field inside the furnace to intersect and xe2x80x9csteerxe2x80x9d the arcs away from the furnace sidewalls. However, this is not an entirely satisfactory solution to the problem of arc deflection because the primary effect of the Stenkvist system is merely to spread the arc blowout effect over a larger radial angle.
In U.S. Pat. No. 5,960,027 issued in 1999 to Kiyohara et al., a DC arc furnace is provided with an auxiliary electromagnetic coil system for controlling arc deflection. One or more U-shaped auxiliary coils are arranged around the furnace shell and are connected in series with the DC electrode power circuit. The auxiliary coils are physically oriented in respective planes so as to generate a magnetic field that will cancel the magnetic field generated by the DC current flowing through the power circuit. Unfortunately, the system described by Kiyohara et al. is not adaptable to AC arc furnaces.
What is needed, then, is an AC arc furnace having an auxiliary system for controlling arc deflection produced by electromagnetic fields generated by multiple electrode current loops inside the furnace.
The AC arc furnace of the present invention includes a furnace shell, which is typically cylindrical, enclosing the furnace chamber. Multiple electrodes are positioned vertically within the chamber, with each of the electrodes connected to an external electrode power circuit to generate and sustain an arc between the electrodes and material to be melted that is placed in a lower portion of the chamber. An auxiliary coil system is connected to a coil power circuit, the auxiliary coil system having a plurality of coils corresponding to the number of electrodes. Each coil is arranged around the furnace shell proximate to a corresponding electrode in a defined coil geometry so that when the coil power circuit is activated, each coil generates an external magnetic field that penetrates the shell to control deflection of the arc of a corresponding electrode.
In a preferred embodiment, each coil is electrically connected in series with its corresponding electrode so that the electrode power circuit also functions as the coil power circuit. Also, lamination structures are positioned outside of and proximate to each of the coils to reduce the magnitude of any external magnetic fields generated by the coils. The furnace shell includes at least one non-magnetic window positioned to facilitate penetration into the furnace of the magnetic fields generated by the auxiliary coil system
Each coil of the auxiliary coil system is arranged around the shell and connected to the coil power circuit so that the external magnetic field applied to the arcs has a magnitude and direction that effectively cancels out an internal magnetic field generated by the arc itself and by the electrode power circuit.